The present invention relates to a glass composition. More particularly, the present invention relates to a glass composition for producing a high performance green glass containing iron oxide. A xe2x80x9chigh performancexe2x80x9d glass is one in which the light transmission of the glass is greatly in excess of the heat transmission thereof.
The light transmission of a glass is of major importance if the glass is for use in automotive vehicles. This is because many countries require any windows in a vehicle forward of the B-post to have a light transmission of at least 70%. The heat transmission is usually denoted by the abbreviation xe2x80x9cDSHTxe2x80x9d which stands for the Direct Solar Heat Transmission. The heat transmission is dependent upon the ability of the glass to absorb radiation in the infra-red region of the electromagnetic spectrum and, to a lesser extent, in the visible portion of the spectrum. In some cases, it is also desirable to absorb radiation in the ultraviolet region of the spectrum. Iron oxide is well known as a colorant in glass compositions and has the advantage that it consists of a mixture of both ferrous and ferric iron. Ferrous iron absorbs radiation in the infra-red and ferric iron absorbs radiation in the ultraviolet region of the spectrum.
For producing high performance glasses, it is obviously desirable to increase the ratio of ferrous iron to ferric iron in the iron added to the glass composition. However, particularly when the glass is to be made by the float process, merely increasing the ferrous iron content is not possible. One problem is that iron is a very good absorber of heat. It absorbs heat in the melting tank and the bottom of the tank therefore cools. In so doing, the glass becomes more difficult to melt and problems of devitrification and silica scum arise. It is possible to use special techniques such as vacuum refining to achieve high ferrous ratios in the glass. However, such techniques are extremely expensive to operate. The more usual way of achieving a high ferrous ratio is to use a reducing agent, such as carbon, in the batch of raw materials. However, the use of carbon can also give rise to the problem of silica scum. In passing, it is pointed out that silica scum is a layer of undissolved silica which lies on the surface of the molten glass and can, in extreme cases, form a raft which extends over the entire surface of the glass.
To minimise this problem, it is customary to add sulphate to the batch. Such sulphate is generally in the form of sodium sulphate or calcium sulphate. However, sulphate is not only an agent which assists in the melting of the glass but also acts as an oxidising agent. Thus, the sulphate tends to react with the carbon leaving less carbon to reduce the ferric iron to ferrous iron. Accordingly, although the addition of carbon appears to be a simple way of increasing the ferrous ratio in the glass, it does not, in practice, achieve this, or at least not to the desired extent.
Most glasses also contain sodium. In many patent specifications, the amount of sodium present is stated to be from 10% to 20%. In practice, however, the amount of sodium present lies within the range of 12% to 13%. There are three main reasons for this. Firstly, the higher the amount of sodium in the composition, the higher the cost of the glass. More importantly, high amounts of sodium in a glass have adverse effects on the viscosity of the glass. The third major problem associated with the use of high amounts of sodium is that the glass made therefrom have low durability which is often evidenced by a staining of the glass which makes it unacceptable to users of the glass.
Calcium is also used in glass but has the disadvantage that, if used in large quantities, it causes devitrification. The approximate upper limit before this occurs is 11% by weight. Magnesium is also used in glass. If the content of magnesium in the glass is reduced, the minimum of the ferrous iron absorption, which normally is centred around 1050 nm in a conventional float glass, is shifted to a longer wavelength. This leads both to an improved light transmission and to a reduced solar heat transmission, both of which are clearly beneficial in a high performance glass. Moreover, magnesium is, generally, an expensive component of the batch and its removal will reduce the cost of the glass. However, magnesium slows the rate of crystal growth in the glass and the removal thereof means that the rate of crystal growth increases which manifests itself in devitrification. Furthermore, magnesium is believed to improve the durability of the glass and it is generally believed that its removal would adversely affect the durability of the glass.
The present invention seeks to provide an iron-containing green glass composition which has a performance in excess of 28%, ideally over 30%, with a ferrous content in excess of 30% but which can be easily melted without the disadvantages outlined hereinbefore.
According to the present invention, there is provided a green glass composition which contains at least 14.5% by weight Na2O, at least 10.5% CaO, and at least 0.5% total iron (measured as Fe2O3) the glass being substantially magnesium-free, the glass thus produced having a ferrous value of at least 30% and a performance (light transmission minus direct solar heat transmission) of at least 28% at at least one thickness of 2.8 mm to 5 mm.
As will be readily apparent to those skilled in the art, the term xe2x80x9cmagnesium-freexe2x80x9d denotes that no magnesium is added to the composition. However, magnesium will be present in the glass as an impurity or a trace element in the batch materials or as a residual carry-over from a previous run on a furnace. In practical terms, therefore, the maximum amount of magnesium present in the composition is unlikely to exceed about 0.2 % by weight of the total composition.
We have surprisingly found that such a composition provides a glass which has a difference between its light transmission and its DSHT of at least 28 percentage points, usually in excess of 30%. Even though the ferrous value is in excess of 30%, such composition may be melted on a float furnace with all of the disadvantages expected by the prior art being obviated or at least minimised.